Modifying display attribute in a board

I'm trying to change an attribute display from off to "value".

Each design component has its own attribute containing the Farnell reference. For manual mounting, I would like to show this attribute on the board layout.

I've tried to make this script:

string strCmd;     // script command text
output("sample.txt","wt")
{
board(B) {
strCmd = "";
B.elements(E) {
    E.attributes(A) {
      if(A.name=="FARNELL")
      {
        string tmp,strAttrValue;
        sprintf(tmp, "ATTRIBUTE %s FARNELL DELETE;\n",E.name);
        strCmd += tmp;
        sprintf(tmp, "CHANGE DISPLAY VALUE;\n");
        strCmd += tmp;
        strAttrValue=A.value;
        sprintf(tmp, "ATTRIBUTE %s FARNELL '%s';\n", E.name, strAttrValue);
        strCmd += tmp;
        }
      }
    }
printf("%s\n",strCmd);
exit(strCmd);
}
}

Unfortunately the attribute must be deleted on a schematic !

If I just add the attribute with same datas and a different display data, it won't be updated.

Have you got any solution to update a display attribute in script ?

Thanks in advance,

Luc